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Old 04-17-2010, 05:44 AM   #21
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Originally Posted by kjoudrey View Post
If turbos spin due to exhaust gas flow, and your at higher rpm (more exhaust flow), then why won't the turbo create boost? The waste gate must be full open to not build boost right? So, does that mean that when the engine is under low load (cruising), there isn't enough vacuum to actuate the waste gate?
Couple of things:

First, turbos spin due to heat energy. Diesel trucks will spool turbos at high RPM going downhill, but we're talking like 5psi, not the 30+ they actually run under throttle.

Second, boost is an arbitrary measurement of intake manifold restriction. Never forget that. It just so happens that it conveniently correlates with the flow rates from a specific turbo, but that's about as far as the usefulness goes. Case in point, you can alter the boost that a supercharger runs by changing the intercooler, intake manifold, or even the camshafts. Turbos are much less prone to this since their method of boost control is pressure based.

The wastegate can be totally closed, but if the throttle plate is closed, the intake manifold will be under vacuum. Only when you open the throttle enough to develop some heat in the exhaust will the turbo spool up and pump enough air into the intake manifold to register positive pressure.

Constant boost of 1-2psi on the highway correlates to a cruise speed of approximately 110mph. An 80mph cruise requires something like 10in.hg to maintain, a little LESS than an equivalent naturally aspirated car since the turbo essentially negates any intake restriction there might have been. Only when you tip into the throttle will you enter boost - it is not entirely RPM dependent.
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Old 04-17-2010, 09:20 AM   #22
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also you can tune the car to cruising afrs at 1-2 psi of boost without much danger
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Old 04-17-2010, 10:14 AM   #23
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My turbo spools very quickly, and I still only see boost if I tip in on steep grades during cruise.

Off idle boost, and still 29mpg combined. Some of it is turbo sizing, the rest is tuning. If you still need a responsive turbo gas mileage is no reason to stay away. My 75mile commute consists of lots of hills, sweepers and roads through the forest from the Lake Conroe area to the Brazos valley. Having a small turbo is great around town.

Now I can see having a bigger turbo for better highway passing power. If you're already at an engine speed that would spool a big turbo then fine. Mine peters off at high rpms, simply runs out of airflow, but it's much smaller than an SR20 T25 even.
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Old 04-17-2010, 02:41 PM   #24
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Thanks for all the help guys! I'm sure I'll post more questions once I start getting parts in hand.
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Old 04-17-2010, 04:54 PM   #25
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Originally Posted by mekaw View Post
lurk around zilvia.net to grab some sr20 t25
or SR20forum.com, that's where I found mine. The T25 from the W10 Avenir SR20 engine fits FM/Bell manifolds, has the .64 A/R, and doesn't have the bolt-on style compressor inlet. There always seem to be a few on SR20forum, got mine for $85 shipped (east coast to west coast) and stuck in a GPopShop rebuild kit for $88.
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